1. Field of the Invention
The present invention relates to a heart-function monitor apparatus which monitors a function of the heart of a living subject by evaluating a blood-outputting ability of the left ventricle of the heart.
2. Related Art Statement
When a characteristic of the left ventricle of the heart as an elastic tube, that is, an elastic coefficient of the same, at a telesystolic time immediately before the aortic valve is closed, is defined as a left-ventricle telesystolic elastance Ees, the elastance Ees indicates a blood-outputting ability of the left ventricle. Accordingly, the elastance Ees can be used as an important index of the function of the heart. For example, the elastance Ees can be used as a quantitative index of the dynamic condition of the circulatory system of a patient under intensive care or anesthesia.
However, determination of the above left-ventricle telesystolic elastance Ees, which is also known as the maximum pressure-volume ratio, or the left-ventricle telesystolic pressure-volume ratio, needs (a) detecting continuously respective changes of the inner pressure and inner volume of the left ventricle, (h) obtaining, in a two-dimensional coordinate system having a volume axis indicative of the inner volume of the left ventricle and a pressure axis indicative of the inner pressure of the same, a plurality of pressure-volume loops before and after preload or afterload is applied to the cardiac muscle, (c) estimating, based on the plurality of pressure-volume loops, a left-ventricle unstressed volume, V0, taken when the inner pressure would take zero, and (d) determining the telesystolic elastance Ees by dividing a telesystolic pressure, Pes, by the difference of a telesystolic volume, Ves, and the unstressed volume V0. Thus, the determination of the telesystolic elastance Ees needs measuring simultaneously the inner pressure and inner volume of the left ventricle. Conventionally, this determination has been carried out by an invasive method in which a cutting operation or a catheter insertion is needed. Thus, it has been very difficult to monitor the cardiac function.
It is therefore an object of the present invention to provide a heart-function monitor apparatus which can non-invasively and easily monitor a left-ventricle telesystolic elastance Ees of a living subject.
The Inventor has carried out extensive studies in the above-mentioned background, and has found the fact that when (a) a pressure-volume ratio, E(t), is obtained by dividing a continuously obtained left-ventricle inner pressure, P(t), by the difference, (V(t)xe2x88x92V0), of a continuously obtained left-ventricle inner volume, V(t), and the above-indicated unstressed volume V0, (b) a time-and-pressure-volume-ratio curve is drawn, as shown in FIG. 8, in a two-dimensional coordinate system having a time axis and a pressure-volume-ratio axis, (c) a first portion of a length of the time-and-pressure-volume-ratio curve between its start end and a maximum pressure-volume ratio, Emax, i.e., a left-ventricle telesystolic elastance Ees (the first portion corresponds to a pre-ejection period, PEP) is approximated by a straight line, L1, and a second portion of the length (the second portion corresponds to an ejection period, ET) is approximated by a straight line, L2, and (d) xcex10 is defined as being equal to the ratio, xcex12/xcex11, of a slope, xcex12, of the straight line L2 to a slope, xcex11, of the straight line L1, the left-ventricle telesystolic elastance Ees can be expressed by using a telediastolic aorta (blood) pressure, Pad, i.e., an aorta inner pressure at a telediastolic time of the heart; a telesystolic aorta pressure Pes, i.e., an aorta inner pressure at a telesystolic time of the heart; the ejection period ET and the pre-ejection period PEP of the left ventricle; a stroke volume, SV, i.e., a volume of the blood outputted by one beat of the left ventricle; a telediastolic left-ventricle pressure, Ped, i.e., a left-ventricle inner pressure at the telediastolic time of the heart; and the ratio xcex10. The present invention has been developed based on this finding.
(1) According to a first feature of the present invention, there is provided an apparatus for monitoring a function of a heart of a living subject, comprising a pre-ejection period measuring device which non-invasively measures a pre-ejection period from a time when contraction of a cardiac muscle of a left ventricle of the heart starts, to a time when ejection of blood from the left ventricle starts; an ejection-period measuring device which non-invasively measures an ejection period during which the blood is ejected from the left ventricle; an aorta-pressure estimating means for estimating blood pressure values in an aorta of the subject; a telediastolic-aorta-pressure determining means for determining, based on the aorta blood pressure values estimated by the aorta-pressure estimating means, a telediastolic blood pressure in the aorta at a telediastolic time of the heart; a telesystolic-aorta-pressure determining means for determining, based on the aorta blood pressure values estimated by the aorta-pressure estimating means, a telesystolic blood pressure in the aorta at a telesystolic time of the heart; a stroke-volume measuring device which non-invasively measures a stroke volume that is a volume of blood ejected from the left ventricle of the heart by a one-time beat of the heart; and a telesystolic-elastance determining means for determining, based on the measured pre-ejection period, the measured ejection period, the determined aorta telediastolic blood pressure, the determined aorta telesystolic blood pressure, and the measured stroke volume, a telesystolic elastance of the left ventricle of the heart, according to a predetermined relationship between (A) left-ventricle telesystolic elastance and (B) (b1) pre-ejection period, (b2) ejection period, (b3) aorta telediastolic blood pressure, (b4) aorta telesystolic blood pressure, and (b5) stroke volume.
According to this feature, the telesystolic-elastance determining means determines, based on the pre-ejection period, the ejection period, the telediastolic aorta blood pressure, the telesystolic aorta blood pressure, and the stroke volume all of which are non-invasively measured or determined, a telesystolic (i.e., end-systolic) elastance of the left ventricle of the heart of the subject, according the predetermined relationship. Thus, the present heart-function monitor apparatus can non-invasively and easily monitor the left ventricular end-systolic elastance corresponding to the cardiac function of the subject.
(2) According to a second feature of the present invention that includes the first feature (1), the telesystolic-elastance determining means comprises means for determining, according the predetermined relationship, the telesystolic elastance of the left ventricle of the heart, based on the measured pre-ejection period, the measured ejection period, the determined aorta telediastolic blood pressure, the determined aorta telesystolic blood pressure, the measured stroke volume, and a predetermined telediastolic (i.e., end-diastolic) pressure in the left ventricle at the telediastolic time of the heart. The left ventricular end-diastolic pressure is, e.g., 10 mmHg, but may be non-invasively estimated by the present monitor apparatus.
(3) According to a third feature of the present invention that includes the second feature (2), the predetermined relationship is defined by a following expression:
Ees=[Pad+{(Padxe2x88x92Ped)/PEP}xc3x97ETxc3x97xcex10xe2x88x92Pes]/SV
where
Ees is the left-ventricle telesystolic elastance,
Pad is the aorta telediastolic blood pressure,
Pes is the aorta telesystolic blood pressure,
Ped is the left-ventricle telediastolic pressure,
ET is the ejection period,
PEP is the pre-ejection period,
SV is the stroke volume, and
xcex10 is a coefficient.
The above expression is obtained based on the fact that when a portion of the time-elastance curve (FIG. 8) between its start end and the maximum elastance Emax, i.e., the telesystolic elastance Ees is approximated by the two straight lines L1, L2, the elastance Ees can be expressed by using the telediastolic aorta (blood) pressure Pad, the telesystolic aorta pressure Pes, the ejection period ET, the pre-ejection period PEP, the stroke volume SV, the telediastolic left-ventricle pressure, and the ratio xcex10 of the slope xcex12 of the line L2 to the slope xcex11 of the line L1.
(4) According to a fourth feature of the present invention that includes the third feature (3), the monitor apparatus further comprises an output-rate measuring device which non-invasively measures a volume of the left ventricle at the telediastolic time of the heart, and determines an output rate of the left ventricle of the heart by dividing the measured stroke volume by the measured left-ventricle telediastolic volume, and the telesystolic-elastance determining means determines, based on the measured pre-ejection period, the measured ejection period, the determined aorta telediastolic blood pressure, the determined aorta telesystolic blood pressure, the measured stroke volume, the measured output rate, and the left-ventricle telediastolic pressure, a telesystolic elastance of the left ventricle of the heart, according a predetermined relationship between) (A) left-ventricle telesystolic elastance and (B) (b1) pre-ejection period, (b2) ejection period, (b3) aorta telediastolic blood pressure, (b4) aorta telesystolic blood pressure, (b5) stroke volume, and (b6) output rate. According to this feature, the output-rate measuring device measures the output rate (i.e., ejection fraction) of the left ventricle that is known as being well correlated to the telesystolic elastance, and the telesystolic-elastance determining means determines the telesystolic elastance of the left ventricle, based on the measured output rate, the measured pre-ejection period, the measured ejection period, the determined aorta telediastolic blood pressure, the determined aorta telesystolic blood pressure, the measured stroke volume, the measured output rate, and the left-ventricle telediastolic pressure. Thus, the telesystolic-elastance determining means determines a more accurate telesystolic elastance of the left ventricle.
(5) According to a fifth feature of the present invention that includes the fourth feature (4), the coefficient xcex10 of the expression is defined by a following expression:
xcex10=C1+C2xc3x97EXP(C3xc3x97EF)
where
EF is the measured output rate,
C1, C2, and C3 are constants which are experimentally obtained, and
EXP(Z) is an exponential function of Z.
(6) According to a sixth feature of the present invention that includes of the fourth feature (4), the coefficient xcex10 of the expression is defined by a following expression:
xcex10=C1+C2xc3x97EXP(C3xc3x97EF)+C4xc3x97EXP{C5xc3x97PEP/(PEP+ET)}
where
EF is the measured output rate,
C1, C2, C3, C4, and C5 are constants which are experimentally obtained, and
EXP(Z) is an exponential function of Z.
According to this expression, the coefficient xcex10 occurring to the expression used according to the third feature (3) is determined based on the measured output rate and a ventricular contraction index, IV (=PEP/(PEP+ET)), that is conventionally known as an index of cardiac contractility. Therefore, the telesystolic-elastance determining means can more accurately determine the left ventricular telesystolic elastance according to the expression used according to the third feature (3).
(7) According to a seventh feature of the present invention that includes any of the first to sixth features (1) to (6), the pre-ejection period measuring device comprises an electrocardiograph which includes a plurality of electrodes adapted to contact a body surface of the subject, and which detects, from the subject, an electrocardiogram waveform including a Q wave; a heart-sound detecting device which is located in a body cavity of the subject, at a position in a vicinity of the heart of the subject, and which detects, from the subject, at least a first heart sound I; and means for determining, as the pre-ejection period, a time period from a time when the Q wave of the electrocardiogram waveform is detected to a time when an end of the first heart sound I is detected. Thus, the pre-ejection period measuring device can non-invasively measure the pre-ejection period PEP with accuracy.
(8) According to an eighth feature of the present invention that includes any one of the first to seventh features (1) to (7), the ejection-period measuring device comprises a heart-sound detecting device which is worn on the subject, at a position in a vicinity of the heart of the subject, and which detects, from the subject, a first heart sound I and a second heart sound II; and means for determining, as the ejection period, a time period from a time when an end of the first heart sound I is detected to a time when a start of the second heart sound II is detected. The ejection-period measuring device can non-invasively measure the ejection period ET with accuracy.
(9) According to a ninth feature of the present invention that includes any one of the first to eighth features (1) to (8), the telediastolic-aorta-pressure determining means comprises an electrocardiograph which includes a plurality of electrodes adapted to contact a body surface of the subject, and which detects, from the subject, an electrocardiogram waveform including a Q wave; and means for determining, as the telediastolic aorta blood pressure, a blood pressure which is estimated by the aorta-pressure estimating means as an aorta blood pressure at a time when the Q wave of the electrocardiogram waveform is detected by the electrocardiograph. The telediastolic-aorta-pressure determining means can non-invasively determine the telediastolic aorta blood pressure with accuracy.
(10) According to a tenth feature of the present invention that includes any one of the first to ninth features (1) to (9), the telesystolic-aorta-pressure determining means comprises a heart-sound detecting device which is worn on the subject, at a position in a vicinity of the heart of the subject, and which detects at least a second heart sound II from the subject; and means for determining, as the telesystolic aorta blood pressure, a blood pressure which is estimated by the aorta-pressure estimating means as an aorta blood pressure at a time when a start of the second heart sound II is detected by the heart-sound detecting device. The telesystolic-aorta-pressure determining means can non-invasively determine the telesystolic aorta blood pressure with accuracy.
(11) According to an eleventh feature of the present invention that includes any one of the first to tenth features (1) to (10), the pre-ejection period measuring device non-invasively measures, each time the heart contracts and expands, a pre-ejection period from a time when the contraction of the cardiac muscle of the left ventricle of the heart starts, to a time when the ejection of the blood from the left ventricle starts; the ejection period measuring device non-invasively measures, each time the heart contracts and expands, an ejection period during which the blood is ejected from the left ventricle starts; the aorta-pressure estimating means estimates, each time the heart contracts and expands, blood pressure values in the aorta of the subject; each time the heart contracts and expands, the telediastolic-aorta-pressure determining means determines, based on the estimated blood pressure values of the aorta, a telediastolic blood pressure in the aorta at a telediastolic time of the heart; each time the heart contracts and expands, the telesystolic-aorta-pressure determining means determines, based on the estimated blood pressure values of the aorta, a telesystolic blood pressure in the aorta at a telesystolic time of the heart; each time the heart contracts and expands, the stroke-volume measuring device measures a stroke volume of the left ventricle; and each time the heart contracts and expands, the telesystolic elastance determining means determines, based on the measured pre-ejection period, the measured ejection period, the determined telediastolic aorta blood pressure, the determined telesystolic aorta blood pressure, and the measured stroke volume, a telesystolic elastance value of the left ventricle of the heart according to said predetermined relationship, and the monitor apparatus further comprises a display device which displays, along an axis indicative of time, the left-ventricle telesystolic elastance values which are successively determined by the telesystolic elastance determining means as the heart successively contracts and expands. According to this feature, since the display device displays a timewise trend of the successively determined left ventricular end-systolic elastance values, a doctor or a nurse, for example, can recognize, when the cardiac function of a patient who is undergoing a surgical operation is lowering, the tendency or direction of change of the cardiac function, from the displayed timewise trend. Therefore, the doctor or nurse can estimate an abnormality of the cardiac function before the cardiac function actually indicates the abnormality.
(12) According to a twelfth feature of the present invention that includes any one of the first to eleventh features (1) to (11), the aorta-pressure estimating means comprises a blood-pressure measuring device which includes an inflatable cuff adapted to be wound around a body portion of the subject, and which measures at least one blood pressure of the subject when an air pressure in the cuff is changed; a pulse-wave sensor which is adapted to be pressed against an artery of the subject via a skin tissue of the subject so as to flatten a portion of a wall of the artery, and which detects a pressure pulse wave transmitted thereto from the artery via the flattened wall portion of the artery and the skin tissue; relationship determining means for determining a relationship between blood pressure and pressure-pulse-wave magnitude, based on at least one blood pressure measured by the blood-pressure measuring device and at least one magnitude of the pressure pulse wave detected by the pulse-wave sensor; and means for calibrating, according to the determined relationship, instantaneous magnitudes of the pressure pulse wave detected by the pulse-wave sensor, and thereby providing a waveform representing the estimated aorta blood pressure values of the subject.